The present invention relates to optical transmission, in particular to arrangements and methods to be used in optical communication systems, such as telecommunication networks or data networks. The invention also relates to an optical communication system implementing wavelength-division multiplexing (WDM).
In telecommunication and datacommunication networks the requirements on a high capacity and a high speed are steadily increasing. It has been realized that capacity as well as speed can be increased through the transmission of information as optical signals through transmission lines in the form of optical fibres. Wavelength-division multiplexing (WDM) is an attractive way to increase the capacity of existing as well as future optical fiber lines. In a system using WDM, multiple optical carriers carrying data signals directed to different destinations are transmitted through a single optical fibre. WDM takes advantage of the huge wavelength range (frequency domain) that is available in an optical fibre through assigning different wavelengths to different channels. Multiplexing devices combining several wavelengths in one and the same device for separation of different wavelengths are then required. For the building of real optical networks, and not only point-to-point links, routing devices are needed to enable an optical signal to be forwarded without intermediate conversion to an electrical signal.
One known wavelength router is the so called Optical Add-Drop Multiplexer (OADM). The basic functionality of an OADM is to separate one wavelength channel from an incoming light signal corresponding to the drop functionality and replacing the wavelength channel by another signal at the same wavelength corresponding to the add functionality. Various devices have so far been proposed which for example are based on Bragg gratings in fibres, which are based on the use of the acousto-optic effect in LiNbO.sub.3. Another known device uses a demultiplexer to separate the channels and 2-by-2 switches are used for the wavelength add-drop selection whereas another demultiplexer is used for recombination of that channel. This has been integrated on an chip in SiO.sub.2 /Si for 16 channels using three arrayed-waveguide demultiplexers and 16 thermo-optic switches. This is for example described in "16-channel optical add/drop multiplexer using silica-based arrayed-waveguide gratings", Electron. Lett., vol. 31(9), Apr. 27, 1995, pp. 723-724, by K. Okamoto et al. Another integrated OADM-device is described in "First InP-based reconfigurable integrated add-drop multiplexer", IEEE Photon. Technol. Lett., vol. 9(2), February 1997, pp. 188-190, by C. G. M. Vreeburg et al, in which a 4-channel integrated OADM in InP is described which only uses one arrayed-waveguides demultiplexer in a loopback configuration and 4 electro-optic Mach-Zehnder switches.
"Novel InP-based phased-array wavelength demultiplexer using a generalized MMI-MZI configuration", Proc. 7th Eur. Conf. on Int. Opt. (ECIO'95), paper WeA2, pp 275-278, by C. van Dam et al and "Optical device with phased array", WO 95/22070, describe a MMI-MZI (Multi-Mode Interference--multiple Mach-Zehnder Interferometer) demultiplexer in which no phase control is enabled and which can not perform any switching. The fabrication tolerances of such a device are very severe which makes the devices difficult to fabricate and almost impossible to fabricate such that they repeatedly meets the requirements. Although some slight phase changes are mentioned, these are merely intended to compensate for fabrication deviations, which is necessary in a device wherein the fabrication tolerances are extremely severe. Furthermore devices are known such as MMI-based switches in AlGaAs/GaAs, "Novel 1.times.N and N.times.N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides", Appl. Phys. Lett., vol. 64(6), Feb. 7, 1994, pp. 684-686, by R. M. Jenkins et al. The switches are however essentially wavelength independent over a large wavelength range. Furthermore various fixed wavelength demultiplexers are known.
Consequently the area has been widely researched and a large number of different devices have been discussed, but they all suffer from different drawbacks such as being too complex, consisting of a number of different elements, difficult to fabricate, being fixed wavelength devices etc.
U.S. Pat. No. 5,526,153 shows an optical channel adding/dropping filter. In this device a fixed wavelength multiplexer is however used and the wavelength cannot be changed. To provide a switching functionality, switches would have to be added giving a complex device which among others is not easy and cheap to fabricate and also not small enough. The wavelength is furthermore set already at the fabrications stage.